Rain Gardens
What are rain gardens? Rain gardens are areas of flowers and small plants constructed in a yard that collect rainwater. Rain gardens are designed to prevent surface water and nutrients such as Phosphorus and Nitrogen from running off into the local sewer system. This allows much-needed water and nutrients to seep into the ground, allowing for water filtration and the replenishing of ground water. Rain gardens also help to prevent soil erosion . Because of the physical and biological processes that occur in a rain garden, the excess water that leaves the garden is much cleaner. How Do Rain Gardens Help the Environment? By filtering out nutrients such as Nitrogen and Phosphorus from storm water runoff, rain gardens act as natural water purifiers. Through lawn fertilization, homeowners deposit large amounts of these nutrients into the local landscape. When heavy rainfall occurs, these nutrients are washed from lawns, down the storm sewer, and into local rivers and watersheds. High Nitrogen and Phosphorus levels in rivers disrupt the natural nutrient flows and cause algal blooms which decrease the oxygen content of the water. This further disrupts the ecosystem, killing fish and causing ‘dead zones’, as observed in the Gulf of Mexico where the Mississippi River meets the sea. Through the way in which they are constructed and through the use of certain plants, rain gardens prevent Nitrogen and Phosphorus from running off the lawn. The nutrients are then absorbed by plants and promote healthy plant growth. How Do I Contruct A Rain Garden Rain gardens are constructed by removing top soil to make a concaved-shaped low point in the yard so water flows into it and is captured. Deciding the location for a rain garden is important. Rain gardens are not best in the naturally lowest point of the yard, but rather at a higher point where water would normally flow toward a storm sewer1. During a rain, observe where in your yard water flows and draw out how you might be able to divert water flowing in multiple directions toward your rain garden. Two types of common rain gardens are the self- contained garden and the under-drained garden1. The self-contained garden is designed to absorb all rainwater runoff and work best when constructed in an area that is much higher than the local water table. If a self-contained garden is in a low-lying area, a heavy rain could completely inundate the garden. Additionally, in wetter months, a self-contained garden might remain flooded. An under-drained rain garden is constructed with a drain in the lowest part of the garden (often the center) that allows for excess water to be diverted to the storm sewer or a rain barrel. Under-drained gardens are more ideal for rain gardens in low-lying areas, but do not capture as much water and nutrients from running off. Once the location of the rain garden has been chosen, be sure to call your local utility company to mark where buried water and electrical lines might be. The rain garden area should be excavated so that it is 8”-12” lower than the adjacent land1. To allow for greater bioretention of water and nutrients, some of the excavated soil could be placed at the edge of the garden on the side where water would normally flow to the storm sewer. The rain garden area should be filled with amendment materials such as a combination of sharp sand, mulch and top soil. The mulch allows for proper water drainage, weed prevention and reduced stress on plants in times of inundation. What Plants Are Best In A Rain Garden? Because the plants in a rain garden are usually native and hearty varieties, they often attract a variety of butterflies and birds and promote biodiversity2. The plants chosen to comprise your rain garden should depend on: how much sun the area receives, plant drought and flood tolerance, the local ecosystem, your desired level of garden maintenance and your aesthetic preferences. For rain gardens in the upper Midwest of North America, native prairie flowers work well as they are more drought and flood tolerant than foreign varieties and require a low level of maintenance2. Furthermore, prairie flowers have extended root structures that allow for better water infiltration. Shrubs, trees and other plant varieties also work in rain gardens as long as they are flood tolerant. Learn More At: 1. http://www.lowimpactdevelopment.org/raingarden_design/whatisaraingarden.htm 2. http://dnr.wi.gov/wnrmag/html/stories/2008/apr08/raingarden.htm